1. Field of the Invention
This invention generally concerns a system and method for remotely plugging an opening in the wall of a vessel to which access is limited. It is specifically concerned with an improved system and method for plugging the flow ports of a core barrel in a pressurized water nuclear reactor which are closely surrounded by a thermal shield.
2. Discussion of the Prior Art
Devices for plugging the core barrels of pressurized water nuclear reactors are known in the prior art. Generally, such prior art devices are comprised of a plug body formed from a cylindrical shell that surrounds a tapered mandrel. One end of the shell is insertable into the mouth of a flow port, while the other end is circumscribed by an integrally formed retaining flange that controls the extent to which the cylindrical shell may be inserted into the port. A tapered mandrel axially movable within the cylindrical shell wedgingly engages tapered walls present in the interior of the shell when pushed therein in order to radially expand the shell into sealing engagement with the flow port, thereby plugging it. The tapered mandrels in such prior art plugging devices are pushed against the tapered walls in the interior of the cylindrical shell of the plug body by means of a pressurized hydraulic fluid. To this end, the plug body includes its own hydraulic chamber at its flanged end, and a nipple or other inlet means for admitting a hydraulic fluid. The tapered mandrel sealingly engages the interior walls of the shell so that when hydraulic fluid is admitted into the chamber present at the flanged end of the shell, the mandrel moves piston-like toward the end of the shell, thereby expanding it.
Such plugging mechanisms are very useful in converting the coolant flow in a reactor vessel from a downflow pattern to an upflow pattern. However, before the overall purpose and utility of such plugging mechanisms may be fully appreciated, a brief discussion of the significance of the upflow conversion of the reactor vessel internals is necessary.
Conventional pressurized water reactors include a reactor vessel having a core barrel disposed therein that produces heat by means of a plurality of nuclear fuel rod assemblies. A water coolant is circulated through the core barrel and in heat transfer relationship with the nuclear fuel assemblies so that heat is transferred from the assemblies to the water coolant. The fuel assemblies are surrounded by an arrangement of vertical metal baffle plates that define the outer limits of the core barrel. Although the baffle plates are joined together to form an outer perimeter for the core barrel, these plates are bolted, rather than welded, together. Consequently, small gaps sometimes exist between two adjacent baffle plates. In some nuclear reactors, the particular path of the water coolant flowing through the core barrel creates a pressure differential which causes high pressure streams of coolant to squirt or "jet" through the gap between the baffle plates and into the core barrel. These streams of coolant will sometimes impinge on the relatively delicate fuel rods contained in the fuel rod assemblies, and cause them to rattle against their support grids. Such rattling may damage and ultimately break the delicate fuel rods, thereby contaminating the water coolant within the core barrel with particles of radioactive uranium oxide.
One particularly successful solution to the problems caused by such coolant "jetting" is disclosed in U.S. Pat. Nos. 4,576,778 and 4,591,068, both of which are assigned to the Westinghouse Electric Corporation. In this solution, the flow ports present in the walls of the core barrel of the reactor are plugged, thereby converting the reactor from a by-pass downflow configuration to a by-pass upflow configuration (see FIGS. 1 and 2). Each of these patents discloses plugging mechanisms that are used to implement such a flowpath conversion in the reactor core by plugging the flowports that normally exist in the walls of the core barrel.
Unfortunately, such prior art plugging mechanisms are not readily usable in all core barrel designs. For example, in some core barrels, the flowports include a broadly chamfered portion that leads into a small diameter opening. Such chamfered, small-diameter flowports are difficult to plug with existing plug mechanisms for three reasons. First, the plug bodies are too short to be inserted deeply enough past the chamfer to create a good sealing engagement between the distal end of the plug body, and the non-chamfered portion of the flowport. However, if one attempts to solve this problem by merely elongating the plug body, insertion into the flowport prior to the expansion of the plug body becomes difficult, if not impossible, due to the mechanical interference created by the thermal shield that is spaced a very short distance away from the chamfered portion of the flowport. A second problem is created by the relatively small diameter of the non-chamfered portion of the port. If one attempts to scale down the diameter of the pressure chamber within the plug body to accommodate the relatively smaller diameter of such flowports, the hydraulic pressures necessary to drive the tapered mandrel into wedging engagement against the tapered inner walls of the cylindrical shell become high enough to subject the inside of the plug body to unacceptably high levels of mechanical stress. The third difficulty results from the fact that both the diameter and the depth of the chamfered portions of such flowports vary. Hence, no single-sized plugging mechanism is capable of adequately plugging all of the different-sized flowports present in such core barrels.
Clearly, what is needed is an improved plugging system whose plugs may be easily manipulated in the limited access space between the core barrel and the thermal shield and readily inserted and expanded into a particular flowport. Ideally, such a plugging system should be capable of installing plugs in flowports having chamfered portions of different diameters and different depths. Finally, such an improved system should be capable of reliably plugging flow holes of relatively small diameter without the creation of unacceptably high levels of mechanical stress within the plug body.